Not to fire the flames of controversy, BUT , I am now able to add some real measurements to this post
I have again fired up the old 175W AB bulb (I was not running it due to heat problems.) It is now located eleven inches above my 20H. The Lux meter I bought from Dez some time ago has a probe that allows me to read the irradiance at the bottom of my tank. To make a long story short, I get a reading of 15,000 Lux at the bottom of the 16" deep tank. I am unable to discern a difference between readings with the glass cover in place or not.
The point is that if I am to go by what John Tullock says in his book, that 10,000 Lux is enough for most organisms I need not worry about the glass cover. If I can get used to the color I don't have to replace the bulb either. Unfortunately I like the looks of the 2X65W PC"s much better.
I almost forgot another measurement. I have a piece of M. digitata that is growing well in my 50. The polyps are fully extended, and the tips are pinkish. The location is under 2X96W PC's under a 1/4 inch glass cover, and 8.5 inches under the light. The meter reads 5000 Lux.
I am providing this information so that you may all draw your own conclusions.
